1. Field of the Invention
The present invention relates to an antenna device and a mobile communication terminal equipped with an antenna device, particularly, to an antenna device housed in the casing of a mobile communication terminal and to the mobile communication terminal equipped with the antenna device.
2. Description of the Related Art
The antenna for a mobile communication terminal is being changed from the type resembling the whip type antenna, which formed a main stream in the past and which is mounted to the communication apparatus such that the antenna can be withdrawn to the outside of the casing of the communication apparatus, to a built-in type antenna. The built-in type antenna can be handled very easily when the antenna is used and stored, compared with the antenna of the conventional type. In addition, the built-in type antenna is advantageous in that the degree of freedom in the design of the casing is increased.
If the casing is miniaturized, the built-in type antenna used in the past is arranged very close to the substrate, with the result that the antenna element is positioned close to the metal portion such as the peripheral circuit so as to lower the impedance of the built-in type antenna. It follows that it is possible for an impedance mismatch to be brought about between the built-in type antenna and the power supply circuit so as to lower the performance of the built-in type antenna.
On the other hand, it is possible to avoid the problem in respect of the lowered impedance noted above in the case of using a balance power supply type antenna such as a rectangular loop type, a folded type dipole antenna. However, it is difficult in principle to set appropriately the impedance value of the balance power supply type antenna. In addition, a balance-imbalance converter is required in the case of supplying an electric power from the substrate. It follows that the balance power supply type antenna gives rise to another problem that the power supply loss is increased. Also, the balance power supply type antenna is disadvantageous over, for example, the dipole type antenna in respect of the antenna gain. Such being the situation, the balance power supply type antenna fails to provide a suitable means for overcoming the above-noted difficulty inherent in the built-in type antenna.
Proposed in the past are antennas called a folded monopole type antenna or a folded type dipole antenna. The constructions of these antennas are disclosed in, for example, “Tanaka et al. (Built-in Folded dipole antenna for Mobile Terminal Device), Pre-lecture theses B-1-197 (page 1, FIG. 1), Electronic Information Communication Institute Japan Meeting, 2003”, “Y. Kim et al. (A Folded Loop Antenna System for Handsets Developed and Based on the Advanced Design Concept)” or “Electronic Information Communication Institute English Theses, Vol. E84-B, pp. 2468-2475, September, 2001, pages 1 to 3, FIG. 1”. The folded monopole antenna denotes an antenna prepared by folding a linear dipole antenna in its central portion such that the folded portions are positioned close to each other so as to permit the prepared antenna to have a length that is half the length of the original dipole antenna. Also, the folded dipole antenna denotes an antenna prepared by forming a short-circuiting portion between the both edge portions of a pair of folded monopole antennas so as to form a closed loop. In this case, an electric power is supplied to a point in the closed loop.
In each of the antennas pointed out above, a transmission line formed of two substantially parallel conductive lines is used as a radiating element. Therefore, the impedance can be controlled by the width or the thickness of the linear element and by the distance between the two conductive lines without depending on the distance from the substrate including a metal portion, as pointed out in (Y. Kim et al. “A Folded Loop Antenna system for Handsets Developed and Based on the Advanced Design concept”, Electronic Information Communication Institute English theses Vol. E84-B, pp. 2468-2475, September, 2001, pages 1 to 3, FIG. 1). In the folded monopole antenna, it is desirable for the distance between the lines on both sides of the folding portion to be sufficiently small, compared with the wavelength. The folded monopole antenna or the folded dipole antenna can prevent un-matching of the antenna impedance that is produced due to the close arrangement between the substrate and the antenna.
In another point of view, the folded dipole antenna is substantially equivalent to an antenna prepared by allowing two linear dipole antennas to be positioned close to each other and by forming a short-circuiting portion in each of the both edges of the two linear dipole antennas. In the folded dipole antenna in which these two linear dipole antennas are allowed to form a half wavelength dipole antenna, the vector of the current flowing into the elements on both sides of each folding point corresponding to the short-circuiting point is reversed. It follows that the folded dipole antenna is substantially equivalent spatially to two half wavelength dipole antennas in which the current vector is excited in the same direction. The particular explanation is given in, for example, “Antenna Engineering Handbook, Ohm Inc. Tokyo, October, 1996, page 112, FIGS. 4.1 and 4.2” or “Uchida, Mushiake (Ultra Short Wave Antenna), Corona Inc. Tokyo, August 1961, paragraph 8.4, FIG. 8.7).
The folded dipole antenna electrically forms a closed loop and, thus, is basically adapted for a balance power supply so as to make it possible to avoid the lowering of the impedance. Such being the situation, it is considered reasonable to understand that the folded dipole antenna is an antenna adapted for the application to a mobile communication terminal as far as the antenna is used under a single frequency.
However, the demands for the antenna used in a mobile communication terminal are diversified nowadays. To be more specific, the antenna for a mobile communication terminal is required to be used not only under a single frequency but also under a plurality of frequencies. The demands for use under a plurality of frequencies are derived from the situation that the broadening in the field of use and the flexibility are more required for the mobile communication terminal. For example, the mobile communication terminal is required to conform with a plurality of communication modes differing from each other in the frequency band. The conventional folded dipole antenna is basically adapted for the balance power supply. Therefore, a problem resides in the folded dipole antenna that it is difficult to allow the mobile communication terminal to be used under a plurality of frequencies by the simple method of, for example, adding an imbalance power supply type antenna so as to permit the power supply circuit to be shared. Also, the size of the folded dipole antenna is larger than that of the monopole type antenna, with the result that, where a balance-imbalance converter is inserted between the balance type power supply circuit and the imbalance type power supply circuit, the power supply line loss is increased.
As pointed out above, the conventional imbalance power supply type antenna for a mobile communication terminal gives rise to the problem that the impedance is lowered by the situation that the antenna is positioned close to the substrate. On the other hand, the conventional folded dipole antenna gives rise to the problem that it is difficult for the antenna to be used under a plurality of frequencies.